The third antitrust complaint against IBM's mainframe business was filed with the European Commission Tuesday, compounding the firm's regulatory problems in Europe.

Recently created TurboHercules, a French firm active in the market for open source mainframe computer software, said in a statement that IBM ties its operating system software to purchases of its mainframe hardware, thus freezing out smaller competitors.

"This conduct prevents TurboHercules from providing its product to mainframe customers desiring an open source solution," said Roger Bowler, chairman of TurboHercules.

IBM hit back, accusing TurboHercules of trying to get a free ride on the back of IBM investments in mainframes.

"TurboHercules is an 'emulation' company that seeks a free ride on IBM's massive investments in the mainframe by marketing systems that attempt to mimic the functionality of IBM mainframes," the company said in a statement.

It likened TurboHercules' behavior to that of firms that "seek to market cheap knock-offs of brand-name clothing or apparel."

The Commission wasn't immediately available to confirm receipt of the complaint.

TurboHercules' move follows similar antitrust complaints with the European Commission from two other small IBM competitors, PSI and T3 Technologies. Microsoft, an arch-rival had a stake in PSI but the firm was subsequently bought out by IBM. Meanwhile, T3 ranks Microsoft among its shareholders.

Last year, the U.S. Department of Justice opened an antitrust probe into IBM's dominance of the mainframe market. That probe was sparked by a complaint from the Computer and Communications Industry Association (CCIA), a trade group that ranks Microsoft among its members.

"Having yet another complaint in Europe -- by an open-source company, no less -- points to a systemic pattern of behavior by IBM directed at anyone who threatens its mainframe monopoly," said Erika Mann, CCIA's executive vice president and head of its European office in Brussels.

"We and our industry sincerely hope that IBM reverses course and honors its previous commitments to both the public and the European government to license whatever patents that IBM claims are necessary to use Hercules on reasonable and non-discriminatory terms," she added.

IBM said in its statement that it is "fully entitled to enforce our intellectual property rights and protect the investments that we have made in our technologies."